Two hundred million years ago the earth consisted of the vast single continent Pangea, surrounded by a great planet-wide sea, notes Charles Mann in this follow-up to 1491. Continental drift tore apart Pangaea, and for millennia the hemispheres evolved wildly diverging ecosystems of plants and animals—until Columbus's arrival in the Americas reunited these long-separate worlds. Historians have dubbed this collision of ecosystems and cultures the Columbian Exchange, and some see it as the single most significant event in human history since the Stone Age; for biologists, however, it ranks as the greatest event since the extinction of the dinosaurs. In this "wonderfully entertaining and subtly balanced book" (New Scientist), Mann begins with the world of microbes and moves up the species ladder to mankind, describing the profound effect this exchanging of species had on the culture of both continents.